To me, the subscriber number means nothing in most cases. I don't care if 12 million people play WoW, I am not going to interact with those 12 million people at the same time, what's important to me is the number of people per server, in games like WAR, where population is important.
If a game has enough players for me to be able to play as I like, it's fine. Right now I gave Champions Online a shot, game in which an instance will hold 100 people. Is that too few? Well it's not like I'm going to need more than 4 to make a party and play. Well, seeing more people around you helps the inmersion into an alive world, but it isn't decisive, I'd say. Most people usually play alone when they wander the world, anyways, and will just see the random bypassers without even noticing (even if these will complain that instanced play is stupid because it takes the "massive" out of MMO).